Add option to override defaultManifestVersion

Add an option to override defaultManifestVersion using environment
variable. The environment variable will be used to override the apex
version locally when developing a desert release feature in
mainline-prod branch. Overriding the apex version in mainline-prod
branch allows us to install apex built in mainline-prod branch to device
running code from git_master which has a higher apex version than
mainline-prod branch.

Bug: b/233608815
Bug: b/196860838
Test: presubmit
Test: go test -v ./build/soong/apex/
Change-Id: I8ac2aae2b6b29da7f09db6edb893e0a0f0691e51